The present invention is directed to a dual phase structured (ferrite and martensite) steel sheet product and a method of producing the same.
Applications of high strength steel sheets to automotive parts, electric apparatus, building components and machineries are currently increasing. Among these high strength steels, dual phase steel, which possess microstructures of martensite islands embedded in a ferrite matrix, is attracting more and more attention due to such dual phase steel having a superior combination of the properties of high strength, excellent formability, continuous yielding, low yield strength/tensile strength ratio and/or high work hardening. Particularly with respect to automotive parts, martensite/ferrite dual phase steels, because of these properties, can improve vehicle crashworthiness and durability, and also can be made thin to help to reduce vehicle weight as well. Therefore, martensite/ferrite dual phase steels help to improve vehicle fuel efficiency and vehicle safety.
The previous research and developments in the field of dual phase steel sheets have resulted in several methods for producing dual phase steel sheets, many of which are discussed below.
U.S. Patent Application Publication No. 2003/0084966A1 to Ikeda et al. discloses a dual phase steel sheet having low yield ratio, and excellence in the balance for strength-elongation and bake hardening properties. The steel contains 0.01-0.20 mass % carbon, 0.5 or less mass % silicon, 0.5-3.0 mass % manganese, 0.06 or less mass % aluminum, 0.15 or less mass % phosphorus, and 0.02 or less mass % sulfur. The method of producing this steel sheet includes hot rolling and continuous annealing or galvanization steps. The hot rolling step includes a step of completing finish rolling at a temperature of (Aγ3−50)° C., meaning (Ar3−50)° C., or higher, and a step of cooling at an average cooling rate of 20° C. per second (° C./s) or more down to the Ms point (defined by Ikeda et al. as the matrix phase of tempered martensite) or lower, or to the Ms point or higher and the Bs point (defined by Ikeda et al. as the matrix phase of tempered bainite) or lower, followed by coiling. The continuous annealing step includes a step of heating to a temperature of the A1 point or higher and the A3 point or lower, and a step of cooling at an average cooling rate of 3° C./s or more down to the Ms point or lower, and, optionally, a step of further applying averaging at a temperature from 100 to 600° C.
U.S. Pat. No. 6,440,584 to Nagataki et al. is directed to a hot dip galvanized steel sheet, which is produced by rough rolling a steel, finish rolling the rough rolled steel at a temperature of Ar3 point or more, coiling the finish rolled steel at a temperature of 700° C. or less, and hot dip galvanizing the coiled steel at a pre-plating heating temperature of Ac1 to Ac3. A continuous hot dip galvanizing operation is performed by soaking a pickled strip at a temperature of 750 to 850° C., cooling the soaked strip to a temperature range of 600° C. or less at a cooling rate of 1 to 50° C./s, hot dip galvanizing the cooled strip, and cooling the galvanized strip so that the residence time at 400 to 600° C. is within 200 seconds.
U.S. Pat. No. 6,423,426 to Kobayashi et al. relates to a high tensile hot dip zinc coated steel plate having a composition comprising 0.05-0.20 mass % carbon, 0.3-1.8 mass % silicon, 1.0-3.0 mass % manganese, and iron as the balance. The steel is subjected to a primary step of primary heat treatment and subsequent rapid cooling to the martensite transition temperature point or lower, a secondary step of secondary heat treatment and subsequent rapid cooling, and a tertiary step of galvanizing treatment and rapid cooling, so as to obtain 20% or more by volume of tempered martensite in the steel structure.
U.S. Pat. No. 4,708,748 (Divisional) and U.S. Pat. No. 4,615,749 (Parent), both to Satoh et al., disclose a cold rolled dual phase structure steel sheet, which consists of 0.001-0.008 weight % carbon, not more than 1.0 weight % silicon, 0.05-1.8 weight % manganese, not more than 0.15 weight % phosphorus, 0.01-0.10 weight % aluminum, 0.002-0.050 weight % niobium and 0.0005-0.0050 weight % boron. The steel sheet is manufactured by hot and cold rolling a steel slab with the above chemical composition and continuously annealing the resulting steel sheet in such a manner that the steel sheet is heated and soaked at a temperature from a→γ transformation point to 1000° C. and then cooled at an average rate of not less than 0.5° C./s but less than 20° C./s in a temperature range of from the soaking temperature to 750° C., and subsequently at an average cooling rate of not less than 20° C./s in a temperature range of from 750° C. to not more than 300° C.
All of the above patents and the above patent publication are related to the manufacture of dual phase steel sheets using a continuous annealing method applied to cold rolled steel sheet. A need is thus still called for to develop a new manufacturing method to produce dual phase steel sheets directly by hot rolling without subsequent cold rolling and annealing to reduce manufacturing processes and corresponding costs. This appears particularly important in North America, where a number of steel manufacturers have no continuous annealing production lines to perform controlled cooling.
The present invention is a hot rolled steel sheet having a dual phase microstructure comprised of a martensite phase less than 35% by volume and a ferrite phase of at least 50% by volume formed in the hot-rolled steel sheet after cooling. As used herein a “hot rolled sheet” and “hot rolled steel sheet” means a steel sheet that has been hot rolled, before cold rolling, heat treatment, work hardening, or transformation by another process. The steel sheet also has a composition comprising carbon in a range from about 0.01% by weight to about 0.2% by weight, manganese in a range from about 0.3% by weight to about 3% weight, silicon in a range from about 0.2% by weight to about 2% by weight, chromium and nickel in combination from about 0.2% by weight to about 2% by weight where chromium if present is in a range from about 0.1% by weight to about 2% by weight and nickel if present is in an amount up to about 1% by weight, aluminum in a range from about 0.01% by weight to about 0.10% by weight and nitrogen less than about 0.02% by weight, where the ratio of Al/N is more than about 2, molybdenum less than 0.2% by weight, and calcium in a range from about 0.0005% by weight to about 0.01% by weight, with the balance of the composition comprising iron and incidental ingredients. Additionally, the steel sheet comprises properties comprising a tensile strength of more than about 500 megapascals and a hole expansion ratio more than about 50% and more particularly may have a tensile strength 590 megapascals and a hole expansion ratio more than about 70%. Alternately, the ratio of Al/N may be more than 2.5, or may be more than about 3.
For hot rolled sheet which is for subsequent processing by cold rolling, alternative steel composition may be provided as above described except the silicon range may be from about 0.05% to about 2%, and the amount of molybdenum may be up to 0.5%.
In various embodiments, the steel composition may have copper in an amount up to about 0.8% by weight, phosphorous in an amount up to about 0.1% by weight, and sulfur in an amount up to about 0.03% by weight. In some embodiments, the composition may additionally include titanium in an amount up to about 0.2% by weight, vanadium in an amount up to about 0.2% by weight, niobium in an amount up to about 0.2% by weight, and boron in an amount up to about 0.008% by weight.
The hot rolled dual phase steel may be made by a method comprising:                (I) hot rolling a steel slab having the above composition into a hot band at a hot rolling termination temperature in a range between about (Ar3−60)° C. and about 980° C. (about 1796° F.);        (II) cooling the hot band at a mean rate of at least about 5° C./s (about 9° F./s) to a temperature not higher than about 750° C. (about 1382° F.); and        (III) coiling the hot band to form a coil at a temperature higher than the martensite formation temperature.        
Alternately, the hot rolling termination temperature may be in a range between about (Ar3−30)° C. and about 950° C. (about 1742° F.).
The steel slab prior to hot rolling may have a thickness between about 25 and 100 millimeters. Alternately, the steel slab may be thicker than 100 millimeters, such as between about 100 millimeters and 300 millimeters, but in such thicker slabs preheating may be needed before hot rolling.
The present dual phase steel has improved weld properties with a more stable microhardness profile between the weld and the heat affected zone adjacent the weld than prior dual phase steels. The microhardness stability of the present dual phase steel provides a difference of less than about 100 HV (500 gf), or alternatively less than 80 HV (500 gf), between the highest hardness on a weld and the lowest hardness on a heat affected zone adjacent the weld, when welded with a conventional gas metal arc welding system such as a metal inert gas (MIG) welding system using 90% argon and 10% carbon dioxide gas.
The hot rolled steel sheet may comprise a dual phase microstructure having a martensite phase between about 3% by volume and about 35% by volume in the hot-rolled steel sheet after cooling, and more particularly from about 10% by volume to about 28% by volume in the hot-rolled steel sheet after cooling. The dual phase microstructure of the steel sheet may have a ferrite phase between about 60% and about 90% by volume or between about 65% and about 85% by volume in the hot-rolled steel sheet after cooling. In addition, the hot-rolled steel sheet may have a yield strength/tensile strength ratio less than about 70%.
The invention is explained in more detail in connection with the accompanying Figures and description set forth below.